The present invention relates generally to the problem of cell selection, for e.g. cell handover, in mobile telecommunication systems, and more particularly to the problem of selecting the optimum cell among cells with differing capabilities due to different air interface modes.
In FIG. 1 is shown a view of a typical mobile system with many cells and a number of mobile stations (xe2x80x9cMSxe2x80x9d). Each of these cells has an associated base station (xe2x80x9cBSxe2x80x9d) which is responsible for radio communication over the air interface to mobile stations in that cell. For a given MS at a given time there is usually one serving cell which is the cell with the base station that the MS is receiving service from so that the MS may receive and transmit communication via the serving base station.
It is characteristic of modern cellular systems that it is possible to switch a call from one base station to another while the mobile station is moving from one cell to another within the mobile communication system. This is termed handover or handoff. In mobile communication systems, the handover process uses measurements made by the mobile station, the serving base station, and/or by surrounding base stations, using these measurements in the handover decision-making process. These measurements can be taken of the quality of the connections, or links, between the MS and the base station or surrounding base stations. xe2x80x9cLink qualityxe2x80x9d measurements include e.g. the raw bit error rate (xe2x80x9cBERxe2x80x9d) and the received signal strength of the various links between the MS and its serving base station or between the MS and the surrounding base stations.
For example, in the mobile communication system known as GSM (xe2x80x9cGlobal System for Mobile Communicationsxe2x80x9d), a mobile station monitors the link quality (e.g. raw BER estimate and received signal strength) of the signal (downlink signal) received from the base station of the serving cell, as well as the link quality in terms of reception level, i.e. the received signal strength, of the downlink signal from the base stations in cells adjacent to the serving cell. In addition, the base station of the cell monitors the quality of the signal (uplink signal) received to the base station from every mobile station that is served by that base station.
Handover then occurs when either the measurement of the MS/BS indicates that the link quality in the currently serving cell is low and a better quality can be obtained from an adjacent cell, or an adjacent cell allows communication with lower transmission levels.
The problem of handover in today""s systems can be summed up by saying that the strategy is to keep the mobile station connected to the xe2x80x9cbestxe2x80x9d cell. The problem of selecting the xe2x80x9cbestxe2x80x9d cell is simple in today""s systems compared with the new systems to be developed over the coming years. These future systems will be based on different radio interface modes (e.g. with different coding and modulation schemes).
An example of developments in these future systems is the gradual introduction of several modulation and coding schemes in existing systems like e.g. GSM. Modulation is essentially the function which imposes the characteristics of the electromagnetic field (e.g. amplitude and frequency) onto a set of rules and the data to be transmitted (which xe2x80x9cmodulatesxe2x80x9d the transmission). In the case of today""s GSM it is the phase of the electromagnetic field which carries the information. It is usual to distinguish the modulation and demodulation on one hand, and the transmission and reception on the other. The first processes transform digital data to and from a low frequency modulated signal, and the second pair of processes transform this low frequency modulated signal to and from the electromagnetic field.
In current GSM systems the modulation method used is Gaussian Minimum Shift Keying (xe2x80x9cGMSKxe2x80x9d). GMSK provides a compromise between a fairly high spectrum efficiency and a reasonable demodulation complexity. In IS-136 today, the modulation scheme is xcfx80/4-Shift Differential Quaternary Phase-Shift Keying (DQPSK).
In the evolution of second generation cellular systems like GSM and D-AMPS, proposed changes have been made to the modulation scheme in order to provide higher bit rates within the same spectrum. There are several proposed schemes. One of these is Differentially-encoded Binary Continuous Phase Modulation (xe2x80x9cDBCPMxe2x80x9d). This is a family of modulation types, one example of which is xcfx80/4-DBCPM, with the advantage of high power amplifier efficiency. Another proposed scheme is Quaternary-Offset-Quadrature Amplitude Modulation (xe2x80x9cQ-O-QAMxe2x80x9d), also known as Offset-16QAM.
A key difference between these different modulation schemes is that they provide users with different data rates. The goal of the evolution of these systems is to increase user bit rate. The result is that the systems will be using different modulation schemes, often in neighboring cells, in order to provide different users more options for which data rate they will use. In addition, differing data rates will often require differing channel coding schemes. Also, several coding schemes may be used for one modulation. As a result, there will be a variety of coding and modulation schemes providing different data rates.
Current handover algorithms set up that link between base station and mobile station which provides the highest link quality. There do exist, though, numerous variations of this base scheme where additional criteria are used, like vehicle speed, as illustrated in WO-9702716 xe2x80x9cMethod for Determining Handover in a Multicellular Communications Systemxe2x80x9d, or interference in other cells, as illustrated in WO-9528808 xe2x80x9cHandover Method and Arrangementxe2x80x9d. However, none of the existing methods address the problem which will exist in evolved cellular systems, e.g. the problem of selecting the best cell for handover between cells with different air interface modes (e.g. with different modulation and coding schemes).
The present invention relates generally to the problem of cell selection, for e.g. cell handover, in mobile telecommunication systems, and more particularly to the problems discussed above. The means of solving these problems according to the present invention are summarized in the following.
As has been seen above, there is a current problem because current handover methods don""t consider the capabilities of different cells in terms of different air interface modes when deciding which cell to handover to. It should also be pointed out that the general problems of selecting the best cell during handover apply also to other situations where cell selection occur, e.g. during call set-up or when the mobile stations continuously select cells during idle mode.
In current mobile communications systems this cell selection is not a problem because current systems typically use only one modulation scheme and one coding scheme in addition to typically using only one carrier and slot. In comparison, new systems are being proposed and developed, e.g. evolutions of GSM. In these evolved systems there will simultaneously exist different systems and different cells, each with different modulation and/or coding schemes to provide different data rates to different users. Present cell selection methods are not optimally designed to select the xe2x80x9cbestxe2x80x9d cell for handover in these evolved systems.
Accordingly, it is one object of the present invention to provide a method of selecting the best cell from the user""s point of view, i.e. in terms of maximizing the Quality of Service (xe2x80x9cQoSxe2x80x9d). The maximum QoS can be given in terms of e.g. higher data/bit rate or throughput. This new method of selecting the best cell is done by extending the known algorithm for cell selection and handover, and then applying additional criteria that take into account the capabilities of mobile station and the base stations that are possible candidates. Preferably, achievable data rate (for xe2x80x9ctransparentxe2x80x9d services) or throughput (for xe2x80x9cnon-transparentxe2x80x9d services) is predicted for the different cell candidates, based on their received signal strength or C/I estimates, their different capabilities, multislot availability, etc., and the cell is selected for which the predicted data rate or throughput is maximum.
For the prediction of data rate and throughput, the different levels of robustness of the air interface modes against noise and interference should be taken into account. This can be done for non-transparent services inherently by estimation of Quality of Service in terms of throughput. For transparent services, the quality of service as given by bit rate and required bit error rate, is a suitable criterion. A strategy for the latter case is to select the xe2x80x9cclosestxe2x80x9d, e.g. measured by the received signal strength on the broadcast channel, cell that provides the required QoS.
By applying the extended cell selection algorithm according to the present invention, data rate or throughput on some connections may be increased at the cost of higher interference. Thus, another embodiment of the present invention is to take into account system measures like e.g. load and estimated interference level. This may be useful in order to enable the operator to allow such far-distance connections with high data rates only if impact on the system is low. This would be true where e.g. the system load is low.
Although the invention has been summarized above, the method according to the present invention is defined according to appended claims 1 and 21. Various embodiments are further defined in dependent claims 2-20 and 22.
Although this invention has been discussed primarily in the context of GSM systems, it is to be understood by anyone skilled in the art that the present invention is equally applicable to other types of systems such as evolved IS-136.